US secretary of state Hillary Clinton has urged protesting loyalists to end street violence in Northern Ireland amid heightening fears of further trouble at demonstrations against a decision to restrict the flying of the Union flag at Belfast City Hall.

A death threat against the east Belfast MP Naomi Long marked a serious escalation in tensions after arson attacks on offices used by her non-sectarian Alliance Party.

Even though Unionist Party leaders, including the Northern Ireland First Minister Peter Robinson, called for a suspension of planned protests until the new year, security chiefs are gearing up for sporadic outbreaks of new violence, especially in the greater Belfast area.

Loyalists have targeted the Alliance Party after blaming them for backing the nationalist SDLP and Sinn Fein to push through a vote to limit the flying of the flag to designated days.

Mrs Clinton who met Mr Robinson and the deputy First Minister Martin McGuinness at Stormont Castle before returning to America at the end of a brief European tour, said the threat on Ms Long's life was absolutely unacceptable.

Mrs Clinton said: "There will always be disagreement in democratic societies, but violence is never an acceptable response to those disagreements. All parties need to confront the remaining challenge of sectarian divisions, peacefully together."

With loyalists threatening another protest in Belfast on Saturday - expected to be the busiest shopping day so far this year - Mrs Clinton said: "People have strong feelings, but you must not use violence as a means of expressing those strong feelings.

"The only path forward is a peaceful democratic one. There can be no place in the new Northern Ireland for any violence. The remnants of the past need to be quickly, unequivocally condemned. Democracy requires dialogue, compromise and constant commitment by everyone to protect the rights of everyone."

Belfast City Hall is now one of many civic buildings across Northern Ireland, including Parliament Buildings, Stormont, where the flying of the flag is limited to designated dates, such as the Queen's Birthday.

Mr Robinson, who claimed the City Hall vote was divisive and provocative, revealed that his Democratic Unionist Party had submitted a motion in a bid to begin a process to extend the number of days the flag is hoisted at Stormont.